After upgrade to 1.12.0_rc3, unable to start containers. Fails with error "Unknown runtime specified default." Based on https://github.com/docker/docker/issues/24343 "We had a breaking change between 1.12.0-RC2 and 1.12.0-RC3 (introduced by #23775). We're discussing what's the best way forward here. In the meantime, recreating your containers will solve the issue." I suggest RC3 is removed as it breaks all previous containers. Work around is to manually downgrade to rc2
I would actually recommend removing rc2 instead -- rc3 is the release with the corrected behavior, and as stated by upstream, affected users of rc2 unfortunately do need to recreate their affected containers.
Docker 1.12.0-rc2 has been removed from the tree.